Hyaluronic acid (hereinafter abbreviated ‘HA’) is a bio-polymeric material where repeat unit comprising N-acetyl-D-glucosamine and D-glucuronic acid is linearly repeated in connection, which polymer plentifully exists in vitreous humor, synovial fluid, connective tissues etc. In this specification, the term ‘HA’ means hyaluronic acid and any of its hyaluronate salts. Hyaluronate salts according to the present invention include but not limited to inorganic salts such as sodium hyaluronate and potassium hyaluronate etc. and organic salts such as tetrabutylammonium hyaluronate etc. Preferable hyaluronate salt of HA according to the present invention is sodium hyaluronate.
HA derivatives have been developed in diverse uses for prevention of adhesion after surgical operation, correction of facial wrinkles, dermal augmentation, tissue engineering, osteoarthritic viscosupplement etc. HA derivatives may largely be classified by water solubility into water-soluble derivatives and water-insoluble derivatives. In the case of water-insoluble derivatives, manufacturing methods of those may largely be thought in two ways: one is to react HA with a compound having one functional group to combine this compound with linear chain of HA while the other is to react HA with a compound having two or more functional groups to make crosslinked HA.
There have been reported in several literature various examples to have synthesized many crosslinked, water-insoluble HA derivatives by using compounds such as divinylsulfone, bisepoxide, bishalide, formaldehyde, etc. having two functional groups.
There have been reported in U.S. Pat. No. 4,582,865 to have used divinylsulfone in order to crosslink HA and in U.S. Pat. No. 4,713,448 to have carried out crosslinking reaction by using formaldehyde. And there has been reported in PCT Patent Publication WO86/00912 an example to have used compound containing epoxy groups in order to crosslink various polysaccharides containing carboxyl group.
There is a report that water solubility of HA derivatives is decreased if carboxyl group of HA is activated in aqueous solution by using EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride) and then reacted with compounds containing one amine group to form amide bond (see U.S. Pat. No. 4,937,270). And there is an example to have crosslinked HA with various polyanionic polysaccharides using EDC (see U.S. Pat. No. 5,017,229).